The invention relates to a storage tank, especially for liquefied gases, which is separated into cells by a considerable number of partitions.
In the future, in view of the emissions burden that is caused by traffic, hydrogen will be used increasingly as a fuel for motor vehicles, aircraft and ships. The storage of hydrogen on board these vehicles is suitably carried out in liquid form, since based on the low density of gaseous hydrogen, the storage capacity would otherwise be only very limited. The hydrogen is therefore cooled to about 25 K and introduced into the storage tank that is in the vehicle at a pressure of 3 to 4 bar.
If a combustion engine is used as a drive assembly, the filling pressure of 3 to 4 bar is just enough for proper operation of the engine. When fuel cells are used to power vehicles, however, at this time a hydrogen supply under a pressure of 10 bar is necessary. Storage of hydrogen at a pressure of 10 bar and a corresponding equilibrium temperature of about 31 K is disadvantageous, however, since the storage capacity clearly drops because the density of the liquid hydrogen decreases with rising temperature.
In practice, therefore, the hydrogen that is stored at a pressure of 3 to 4 bar is first compressed to 10 bar, before it is fed to the fuel cell. The pressure increase can be achieved by, for example, introducing additional gaseous hydrogen into the storage tank or by evaporating a portion of the liquid hydrogen. Such a device for pressure build-up with use of a gaslift, which carries liquid hydrogen upward into the gas chamber and on whose upper end an evaporator-heating system to evaporate the liquid hydrogen is provided, is known from, for example, DE 42 12 626 A1.
After the pressure is increased, however, the liquid and the gaseous hydrogen in the storage tank are in a state of thermal imbalance, since the liquid has a temperature of about 25 K, while in the gas atmosphere, a temperature of 31 K exists. The thermodynamic system present in the tank is therefore induced by return condensation of hydrogen gas to create a balance of the thermal ratios of gas and liquid.
The return condensation and the pressure reduction that result from this is prevented in that at least the topmost liquid layer is brought into thermal equilibrium with the gas and the latter is maintained. Heating the entire liquid is undesirable, however, for the reasons of reduced storage capacity that are mentioned above.
A feature of this invention is therefore to provide a device of the above-mentioned type that makes it possible to maintain a temperature range that exists in the liquid in the storage container.
This feature is achieved according to the invention by partitions consisting of a low heat-conduction material, the maximum diameter of the cells in a plane perpendicular to the partitions being less than 50 mm.
According to the invention, the inside of the storage container is separated into individual cells by a considerable number of partitions, by which the liquid transport inside the storage container, in a plane that is perpendicular to the partitions, is greatly limited. The interval of the individual partitions is selected so that even within the individual cells, i.e., in a plane that is parallel to the partitions, movement of liquid are suppressed. It has been shown that it is necessary for this purpose to limit the maximum expansion of cells to 50 mm.
The expansion of the cells in a plane that is perpendicular to the partitions is preferably kept even less than 10 mm, especially preferably less then 5 mm. Based on the use of partitions in the storage container and the dimensioning according to the invention of the cells that result from this, it is therefore ensured that a liquid that is stored in the storage container is kept in the temperature range that is already present or specifically created.
Heat exchange between the partial liquid amounts that are stored in adjacent cells is reduced according to the invention because the partitions consist of a low heat-conduction material. The partial liquid amounts that are stored in the individual cells are thus substantially decoupled thermally. The partitions advantageously do not tightly seal with the walls of the storage container. Consequently, the liquid in each cell can flow at the joint between the partitions and the container wall from one cell into the adjacent cell. This liquid-side compound of the individual cells makes it possible to provide common feeders and discharge pipes as well as control devices, such as, e.g., liquid level sensors, for all cells.
It has been shown that it is advantageous to provide partitions that are made of plastic or a tissue that is impregnated in resin or a similar substance, since these materials are low heat-conductivity materials, light, reasonably priced and easy to process. The partitions are either completely liquid-impermeable, or they have openings that allow only a limited passage of liquid through the partitions.
The cells advantageously have a triangular, quadrilateral or hexagonal cross-section, since, on the one hand, the storage container can be filled uniformly with the latter, and, on the other hand, such cells are relatively simple to produce. In this connection, several partitions are placed against one another, bonded with one another at certain intervals, and then the unbonded areas are non-contiguous, so that cells are produced. The use of cells with round cross-sections is also advantageous for reasons of manufacturing technology.
The partitions are preferably arranged in a basically perpendicular orientation, so that narrow, high cells are produced. In this case, the cells preferably extend from the bottom of the storage container up to a height of at least 80% of the height of the storage container, especially preferably up to a height of between 80 and 95% of the height of the storage container, so that only a small part of a header portion of the container (the remaining 5 to 20%), in which there is gas, has no internal structure.